The polymerization of phosphonitrilic chlorides has been known for a long time; for example, see Audrieth et al, Chem. Rev. 32, pages 119-122 (1943). That reference discloses "phosphonitrilic chlorides form as complete a polyhomologous series as is known in the realm of chemistry". Besides mentioning the compounds [PNCl.sub.2 ].sub.n where n=3 to 7, it refers to various polymeric types, viz, oil, gums, waxes, inorganic rubber, and an infusible non-elastic material.
The polymerization of phosphonitrilic chlorides to high polymers has been subjected to intensive study; confer Chapters 15 and 16 of Allcock, H. R., Phosphorus-Nitrogen Compounds, Academic Press, New York, N.Y. (1972).
Although the elastomeric and high temperature properties of high molecular weight phosphonitrilic chloride have attracted wide attention, use as an everyday plastic has been hampered by hydrolytic instability; Allcock, supra, pages 354 et seq; and Allcock, Scientific American, pages 66-74 (71) (March, 1974). As mentioned by these references, the hydrolytic sensitivity is associated with phosphorus-halogen bonds, and replacement of the halogens with non-hydrolyzable organic groups improves the stability to moisture.
As further discussed by the Allcock references mentioned above and Selwyn H. Rose U.S. Pat. No. 3,515,688, useful polymers with suitable organic groups can be prepared from soluble, high molecular weight phosphonitrilic chlorides. Directions for forming such materials are given in Example 1 of Rose and on pages 309 and 310 of Allcock's book, supra.
The process of our invention is admirably suited for the preparation of these materials. Thus, our process provides high molecular weight phosphonitrilic chloride preparations which are soluble in solvents such as benzene, toluene and tetrahydrofuran. Our polymers can be reacted to form fully, or substantially fully substituted organophosphazenes such as those described by Rose, supra, and in Allcock's book, pages 354-361.
Phosphonitrilic chloride polymers which give rise to fully substituted organophosphazenes were first reported in 1965 and 1966; confer Allcock et al, J. Am. Chem. Soc. 87, 4216 (1965); Inorg. Chem 5, 1709 (1966). For the desired soluble phosphonitrilic chloride precursor, much care is expended in its preparation. For example, reference is made to pages 309-310 of Allcock's book, supra. The starting trimer is recrystallized and decolorized. Small amounts of solvent, H.sub.2 O, oxygen, PCl.sub.5 and linear phosphazene are carefully excluded since they may inhibit the polymerization.
However, even when all such precautions are taken, it is quite common that the phosphonitrilic chloride does not undergo polymerization as desired. Thus, it is recognized that the polymerization procedure can give undesired results inasmuch as the ultimate product possesses properties "which are not as predictable as would be desired"; confer Halasa et al, U.S. Pat. No. 3,829,554, directed to a preliminary heating step which affords a more predictable polymerization.
Our invention can be used to afford a more predictable polymerization. Furthermore, it can be used to prepare a desired soluble high molecular weight phosphonitrilic chloride from a starting material which does not yield acceptable product when polymerized by a prior art method. This is an important aspect of our invention. We are unaware of any other polymerization process which can be used to prepare suitable product from what would otherwise be unsuitable starting material.
Our invention comprises conducting the polymerization reaction in the presence of a very small amount of a halide acceptor such as aluminum chloride. It is surprising that our process yields the desired phosphonitrilic chloride polymer rather than unwanted product. In this regard, reference is made to Paddock, U.S. Pat. No. 3,026,174, which teaches that oil products which are chains of 10-20 PNCl.sub.2 units endcapped with the elements of PCl.sub.5 or a solid rubber product are formed from trimeric phosphonitrilic chloride and amounts of PCl.sub.5 at least ten times greater than what we use to obtain a catalyst effect with AlCl.sub.3. Paddock reports in examples 5 and 6 that using AlCl.sub.3 at 380,000-1,150,862 parts per million leads to formation of a thick black oil. In addition, Bode et al, Ber. 75, 215 (1942) teaches reaction of AlCl.sub.3 with PNCl.sub.2 trimer to yield (PNCl.sub.2).sub.3 2AlCl.sub.3.
Furthermore, the inventive nature of our process is borne out by page 317 of Allcock's book, supra, which teaches AlCl.sub.3, PCl.sub.5 and other substances do not behave as catalysts in the polymerization of (NPCl.sub.2).sub.3.